Abstracts Evidence suggests that adolescents want and need timely, accessible, interactive, personal, and private health information, including information and resources on dating and casual/hook-up relationships. Adolescents seeking relationship information, advice and support connect with friends and others using technology as the majority own smartphones or tablets and use them heavily. The study will address Objective B highlighted in the RFA. We will adapt and examine the effectiveness of myPlan, a web-based app for healthy relationships and safety planning to prevent serious and lethal physical and sexual violence in dating relationships and casual/hook-ups (i.e., defined as intimacy with a member of the preferred sex that is not consider a significant other) with male and female adolescents (ages 15-17 years) living in urban and rural communities. For many, their first experience of violence is in a dating or casual relationship/hook-up with a partner or acquaintance during adolescence. We are focusing on prevention with 15-17 year olds as an important age to build awareness and understanding of healthy relationships and safety planning in unsafe relationships as many are making the transition to independence and seeking intimacy in relationships. Further, evidence is clear that vulnerability of dating/partner violence, especially sexual violence is highest between the ages of 18-24. Therefore, collaboration with younger adolescents is a critical to prevention of severe and lethal intimate violence now and in the future. Our study is conducted in two phases with a multidisciplinary team of researchers and adolescent health advocates and includes an adolescent advisory committee. In Phase 1 (Year 1), we will use the ADAPT-ITT framework, recommended for adaptation of evidence-based HIV interventions but relevant to this study, allowing triangulation of multiple forms of data to inform adaption, document the adaptation process, and balance fidelity to the original app while adapting to the US urban and rural adolescent population, targeting ages 15-17 years. In Phase 2, we will examine the effectiveness of the adapted myPlan app with a longitudinal mixed-methods randomized trial, comparing myPlan to a `usual care' app on primary outcome (prevention/reduction of victimization/perpetration of dating violence, including severe physical and sexual violence) and secondary outcomes (e.g. safety behaviors, bystander behavior, self-efficacy for harm reduction and mental health) with urban and rural male and female adolescents. We plan to enroll 600 male and female adolescents and follow them over 12-months using a validated measure of risk for severe and lethal violence to determine the effectiveness. The findings will be informed for dissemination by the adolescent advisory committee and will include strategies for integrating the myPlan app in established education programs and policies as a resource for adolescents and service providers to advance awareness of healthy relationships and safety planning to prevent violence in intimate relationships.